It acts exactly like a credit card... The only thing that you need to know is your balance. If you dont spend it all at once, you will have a balance that has to be used to the penny. For instance... if you received a $50 gift card and you spend something for $30, and you try and use it in the next store for another $30, you would have to tell them just charge 20 to the card and pay the difference of the 10.
Again because it acts like a credit card, they do not know what your balance is going to be and will just charge the amount in its entirity.. if its over it will not go thru (51.00 will be denied if you dont tell them its a $50 gift card) they cant go over the amount that the card is.